Transformer octave fuzz, 1970s style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Tycobrahe Octavia.

Three transistors in a feedback loop drive a small transformer. Two germanium diodes on its center-tapped secondary rectify both halves of the wave, doubling the pitch of what comes out.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. The two secondary halves swing opposite ways. One diode passes the positive half of the wave, the other the negative half turned right side up. What comes out is close to a full-wave rectified copy of the signal, its fundamental an octave above the note. It’s clearest on single notes above the 12th fret on the neck pickup, where the signal is closest to a sine. Probe SA and SB to see the two halves, then RO. Boost sets the loop gain, and with it how square the wave gets before it’s rectified. The diodes pass nothing until the signal clears their knee, so as a note dies away the octave breaks up into a sputter. Q1 is fitted collector for emitter, as the drawing shows. Switch on “Q1 turned around” to fit it the usual way.

Download.

Parts and what they do

Stock values for the 1.5 kΩ : 600 Ω transformer version. Tap a part on the schematic to change it.

C2, 100 nF
Input coupling cap. Against the bias divider’s 400 kΩ or so, the corner is a few hertz, so it passes the whole guitar.
C3, 150 pF
Small cap from the base to ground against radio pickup. With the guitar’s own resistance it trims the very top.
R1, 180 kΩ
Feeds the bias divider from the battery through a filter.
C1, 33 µF electrolytic
Filters the supply to the bias divider, so battery noise and the circuit’s own current swings stay off Q1’s base.
R2, 820 kΩ
Top of Q1’s bias divider.
R3, 680 kΩ
Bottom of Q1’s bias divider. The base sits slightly under half the battery.
Q1, MPSA18
An NPN fitted collector for emitter, as the drawing shows the original. In this positive-ground circuit that makes the base-collector junction the one that conducts, so it runs backwards: the collector lead acts as the emitter, and the current gain is a few instead of hundreds. The feedback loop still sets the operating point, but the stage has much less gain than the part number suggests.
R4, 1 kΩ
Between Q1’s emitter end and the feedback junction. With the Boost pot it sets how much of the feedback gets shunted to ground for signal.
R7, 47 kΩ
Feedback from Q3’s collector back to Q1. It sets the DC operating point of all three transistors, and against R4 and the Boost pot the signal gain, about 47 kΩ over 1 to 2 kΩ.
C5, 100 µF electrolytic
Couples the Boost pot to the feedback junction, so the pot changes the signal gain without moving the bias.
Boost, 1 kΩ pot, reverse audio taper
A 1 kΩ reverse-audio pot, a variable resistor to ground. Turning it up drops the resistance, shunts more of the feedback away and raises the gain.
R5, 220 kΩ
Load for Q1, from Q2’s base to ground.
R6, 220 Ω
In series with C4, so the treble cut levels off instead of falling on and on.
C4, 1 nF
Rolls off the top at Q2’s base and keeps the loop from oscillating at radio frequencies.
Q2, MPS6519
PNP emitter follower. It buffers Q1’s high-impedance output so it can drive Q3’s base.
R8, 22 kΩ
Emitter resistor of Q2.
Q3, MPS6519
PNP common-emitter stage, where most of the loop’s gain comes from. Its collector drives the transformer and feeds back to Q1 through R7.
R10, 470 Ω
Q3’s emitter resistor. It keeps the loop steady at DC.
C7, 220 µF electrolytic
Bypasses R10, so Q3 runs at full gain for signal.
R9, 1.2 kΩ
Q3’s collector load. Through C6, the transformer primary sits across it for signal.
C6, 33 µF electrolytic
Couples Q3’s collector to the transformer, so no DC flows in the primary.
Primary, 570 mH
Inductance of the transformer primary. Below where its reactance drops to about Q3’s load, it shunts the signal, so a small transformer thins out the bass.
Primary R, 65 Ω
Resistance of the primary winding.
Sec. A, 49 mH
Top half of the center-tapped secondary, from the center tap (ground) to D1. The dot marks the end that swings with the primary’s dotted end.
Sec. A R, 22.5 Ω
Resistance of the top half.
Sec. B, 49 mH
Bottom half of the secondary, from the center tap to D2. It’s wound the same direction, so its far end swings opposite to the top half’s.
Sec. B R, 22.5 Ω
Resistance of the bottom half.
P–A, coupling factor 0.990
Coupling between the primary and the top half. At 1, all the flux links both windings. Below 1, the rest shows up as leakage inductance in series with the windings.
P–B, coupling factor 0.990
Coupling between the primary and the bottom half. Set it differently from the top half’s coupling to unbalance the two halves.
A–B, coupling factor 0.990
Coupling between the two halves of the secondary.
D1, 1N270 germanium
Germanium diode. It passes the top half’s positive swings to the output.
D2, 1N270 germanium
Germanium diode. It passes the bottom half’s positive swings, which are the input’s negative half turned right side up.
Volume, 500 kΩ pot, audio taper
Output level, and the only load on the diodes: 500 kΩ to ground, with the amp’s input across the lower part.

Versions

1.5 kΩ : 600 Ω transformer
Positive ground, so every voltage in the pedal reads negative. The original transformer is known only by its winding resistances (180 Ω and 16 Ω, center tapped). This version uses published figures for a small 1.5 kΩ : 600 Ω interstage transformer with the 600 Ω winding center tapped: 570 mH and 65 Ω on the primary, 195 mH and 45 Ω across the secondary. The 0.99 coupling is a guess. Catalog figures, not a measured transformer.
10 kΩ : 10 kΩ transformer
The same circuit with a 1:1 10 kΩ transformer, a common substitute, using published figures for one such part: 5 H and 550 Ω on the primary, 5 H and 500 Ω across the center-tapped secondary. Each half of the secondary has half the primary’s turns, so the octave comes out at about half the primary’s swing. Catalog figures, not a measured part.

Sources

  • Gottfried Divos, drawing of the circuit (2005)

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.